[New Feature] I2C position encoder support (#6946)

* [New Feature] I2C position encoder support

I plan to continue improving/cleaning this up, as there areas that need work.

* let the cleanups begin.

* progress

* more progress

* comments, rename files, etc.

* clean

* Cleanups per thinkyhead

* a few more cleanups

* cleanups, bugfixes, etc.

* remove unnecessary passes_test(), additional cleanups/optimizations

* cleanups

* misc.

* Fix up I2CPEM.init() and a few other things.

* organize, fix, rename, etc.

* more optimization

* a few more tweaks

Marlin/SanityCheck.h

@@ -270,11 +270,24 @@
   #endif
 #endif
 
+/**
+ * I2C Position Encoders
+ */
+#if ENABLED(I2C_POSITION_ENCODERS)
+  #if DISABLED(BABYSTEPPING)
+    #error "I2C_POSITION_ENCODERS requires BABYSTEPPING."
+  #endif
+
+  #if I2CPE_ENCODER_CNT > 5 || I2CPE_ENCODER_CNT < 1
+    #error "I2CPE_ENCODER_CNT must be between 1 and 5."
+  #endif
+#endif
+
 /**
  * Babystepping
  */
 #if ENABLED(BABYSTEPPING)
-  #if DISABLED(ULTRA_LCD)
+  #if DISABLED(ULTRA_LCD) && DISABLED(I2C_POSITION_ENCODERS)
     #error "BABYSTEPPING requires an LCD controller."
   #elif ENABLED(SCARA)
     #error "BABYSTEPPING is not implemented for SCARA yet."